Mesothelin is a glycoprotein present on the surface of cells of the mesothelial lining of the peritoneal, pleural and pericardial body cavities. It was originally purified from the human pancreatic cancer cell line HPC-Y5 and was shown to have megakaryocyte potentiating ability and hence named megakaryocyte potentiating factor (MPF) (Yamaguchi et al. (1994) J. Biol. Chem. 269:805-808). The mesothelin cDNA was cloned from a library prepared from the HPC-Y5 cell line (Kojima et al. (1995) J. Biol. Chem. 270:21984-21990). The cDNA also was cloned using the monoclonal antibody K1, which recognizes mesotheliomas (Chang and Pastan (1996) Proc. Natl. Acad. Sci. USA 93:136-40). Structurally, mesothelin is expressed on the cell surface as a 60 kDa precursor polypeptide, which is proteolytically processed into a 31 kDa shed component (corresponding to MPF) and a 40 kDa membrane bound component (Hassan et al. (2004) Clin. Cancer. Res. 10:3937-3942).
In addition to being expressed on normal mesothelial cells, mesothelin is overexpressed in several types of human tumors, including all mesotheliomas, many ovarian and pancreatic cancers, and some stomach, lung and endometrial cancers. For example, mesothelin is expressed on approximately 70% of all ovarian cancers, approximately 82% of papillary, serous adenocarcinomas, approximately 83% of all pancreatic adenocarcinomas and approximately 86% of all ductal pancreatic adenocarcinomas.
Mutant mice have been prepared in which the mesothelin gene was disrupted by homologous recombination (Bera, T. K. and Pastan, I. (2000) Mol. Cell. Biol. 20:2902-2906). No anatomical, hematologic or reproductive abnormalities were detected, indicating that mesothelin function is not essential for growth or reproduction, at least in those mice.
Mesothelin specifically interacts with CA125 (also known as MUC-16), a mucin-like glycoprotein present on the surface of tumor cells that previously had been identified as an ovarian cancer antigen. Further, binding of CA125 to membrane-bound mesothelin mediates heterotypic cell adhesion and CA125 and mesothelin are co-expressed in advanced grade ovarian adenocarcinoma (Rump, A. et al. (2004) J. Biol. Chem. 279:9190-9198). Expression of mesothelin in the lining of the peritoneum correlates with the preferred site of metastasis formation of ovarian cancer and mesothelin-CA125 binding is thought to facilitate peritoneal metastasis of ovarian tumors (Gubbels, J. A. et al. (2006) Mol. Cancer. 5:50).
In view of the foregoing, additional agents for modulating the activity of mesothelin are of interest.